uneven on gel electroforesis

i had a problem which is i got uneven on the mid of gel electrophoresis, when running gel electrophoresis we can see the uneven in the middle of gel, after stain the gel, the band also follow the uneven of the loading,
why these is happen.

I have encountered this problem too. It's SDS-PAGE gel. According to many protocols, I supposed to add a layer of water/butanol on top of separation gel right after pouring mixture into the apparatus. However, no matter how carefully or slow I added the water layer, the water drop would fall and mix with the separation gel, causing the uneven surface. Is there any tips on that?

I have encountered this problem too. It's SDS-PAGE gel. According to many protocols, I supposed to add a layer of water/butanol on top of separation gel right after pouring mixture into the apparatus. However, no matter how carefully or slow I added the water layer, the water drop would fall and mix with the separation gel, causing the uneven surface. Is there any tips on that?